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POPerformance surprise with "as" and nullable types
 

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    <p>I'm just revising chapter 4 of C# in Depth which deals with nullable types, and I'm adding a section about using the "as" operator, which allows you to write:</p> <pre><code>object o = ...; int? x = o as int?; if (x.HasValue) { ... // Use x.Value in here } </code></pre> <p>I thought this was really neat, and that it could improve performance over the C# 1 equivalent, using "is" followed by a cast - after all, this way we only need to ask for dynamic type checking once, and then a simple value check.</p> <p>This appears not to be the case, however. I've included a sample test app below, which basically sums all the integers within an object array - but the array contains a lot of null references and string references as well as boxed integers. The benchmark measures the code you'd have to use in C# 1, the code using the "as" operator, and just for kicks a LINQ solution. To my astonishment, the C# 1 code is 20 times faster in this case - and even the LINQ code (which I'd have expected to be slower, given the iterators involved) beats the "as" code.</p> <p>Is the .NET implementation of <code>isinst</code> for nullable types just really slow? Is it the additional <code>unbox.any</code> that causes the problem? Is there another explanation for this? At the moment it feels like I'm going to have to include a warning against using this in performance sensitive situations...</p> <p>Results:</p> <blockquote> <p>Cast: 10000000 : 121<br> As: 10000000 : 2211<br> LINQ: 10000000 : 2143 </p> </blockquote> <p>Code:</p> <pre><code>using System; using System.Diagnostics; using System.Linq; class Test { const int Size = 30000000; static void Main() { object[] values = new object[Size]; for (int i = 0; i &lt; Size - 2; i += 3) { values[i] = null; values[i+1] = ""; values[i+2] = 1; } FindSumWithCast(values); FindSumWithAs(values); FindSumWithLinq(values); } static void FindSumWithCast(object[] values) { Stopwatch sw = Stopwatch.StartNew(); int sum = 0; foreach (object o in values) { if (o is int) { int x = (int) o; sum += x; } } sw.Stop(); Console.WriteLine("Cast: {0} : {1}", sum, (long) sw.ElapsedMilliseconds); } static void FindSumWithAs(object[] values) { Stopwatch sw = Stopwatch.StartNew(); int sum = 0; foreach (object o in values) { int? x = o as int?; if (x.HasValue) { sum += x.Value; } } sw.Stop(); Console.WriteLine("As: {0} : {1}", sum, (long) sw.ElapsedMilliseconds); } static void FindSumWithLinq(object[] values) { Stopwatch sw = Stopwatch.StartNew(); int sum = values.OfType&lt;int&gt;().Sum(); sw.Stop(); Console.WriteLine("LINQ: {0} : {1}", sum, (long) sw.ElapsedMilliseconds); } } </code></pre>
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    1. USJon Skeet
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    1. COWhy not look at the jitted code? Even VS debugger can show it.
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      1. USAnton Tykhyy
    2. COI'm just curious, did you test with CLR 4.0 as well?
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    3. CO@Anton: Good point. Will do at some point (although this isn't in VS at the moment :) @divo: Yes, and it's worse all round. But then that's in beta, so there may be a lot of debugging code in there.
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